1. Field of the Invention
The present invention relates to a method for setting a data transmission rate in a mobile communication system, and more particularly to a method in which data transmission rate is varied depending on the wireless environment.
2. Background of the Related Art
An important issue in wireless Internet service is the data transmission rate. Currently, mobile communication providers provide mobile communication service at a data transmission rate of 14.4 Kbps (IS95A) and 57.6 Kbps (IS95B). IS95B transmits data at a rate that is four times faster than IS95A. If IS95A and IS95B are used, the quality of service provided to users is quite different. IS95A has the same quality as the mobile communication sending short messages of about 40 letters while IS95B transmits data at 57.6 Kbps since the IS95B has base transmission station channels in parallel.
The channels for data transmission in mobile communication systems below IS95B improved the data transmission rate by using supplemental code channels (hereafter referred to as SCCH as well as a fundamental channel (hereafter referred to as FCH). However, IS95C (IS-2000) was introduced and supplemental channel (hereafter referred to as SHC) can be assigned so that the data transmission rate was increased tenfold over communications that use only FCH. This system is referred to as CDMA2000-1X service.
FIG. 1 shows a mobile communication system for packet data services. As illustrated in FIG. 1, the mobile communication system consists of a mobile station 110, a mobile exchange system 120 and a data core network (DCN) 130 having a packet data serving node (PDSN) 131. The mobile exchange system 120 includes a base transmission station (BTS) 121 for controlling the wireless call from the mobile station 110, a base station controller (BSC) 122 for providing the link for transfer of point-to-point protocol (PPP) link data and managing the wireless resource, a packet control function (PCF) 123 for packet connection service and a mobile switching center (MSC) 124. The packet data serving node 130 interfaces between the two networks, a wireless access network (RAN), and the DCN 130.
An operation of the related art mobile communication system for high speed packet data service will be described as follows. The mobile station (MS) 110 requests data service using PPP from the base transmission station (BTS) 121 of the mobile exchange system 120. The base transmission station 121 determines whether the data service requested by the mobile station 110 is a normal request. If the data service requested by the mobile station 110 is a normal request, the base transmission station 121 negotiates a PPP with an upper PDSN 131 through a basic wireless call setup. In this case, the base transmission station 121 is under the control of the base station controller 122. A packet control path is set to a PDSN 130 by a packet controller 123. After PPP negotiation, Internet Protocol (IP) to connect to external data core network (DCN) 130 is allocated to the mobile station 110, and the mobile station 110 is ready to use the data service. The mobile station 110 connects to the external data network and can request and receive the data from the corresponding network.
In other words, if the mobile station 110 using IS95C (CDMA 2000) packet data service requests a packet service, the PDSN 131 to transmit the packet data is determined by the BSC/PCF 122/123. At this time, a wireless traffic channel and the radio link protocol (RLP) Link are set between the mobile station 110 and the BSC 122. The A8/A9 traffic link to transmit the data of the PPP link between the mobile station 110 and the PDSN 130 is set between the BSC 122 and the PCF 123. The A10/A11 RLP link to transmit the data of the PPP link between the mobile station 110 and the PDSN 131 is set between the PCF 123 and the PDSN 131. Data transmission and reception can then be performed.
In order to provide the wireless data service, the Radio Link Protocol (RLP), which is a standard of wireless data defined above, is used. In a wireless environment, bit errors are proportionately higher as compared to a wire environment. When a bit error occurs, data transmission is maintained using RLP based on a Negative Acknowledgment (NAK) and retransmission. For high speed data service, the transmission rate is proportional to the amount of data required by the mobile station, and the transmission path should be determined according to data flow and movement of the mobile station.
The data transmission rate of wireless data calls using conventional PPP is determined by ascertaining data flow and by using the flow control in the upper PPP layer. In other words, since the data transmission rate of the wireless environment is controlled variably only by the amount of data, the efficiency of the wireless data transmission is decreased by a variety of factors that can be generated in a wireless environment. These include transmission output, electromagnetic interference, and NAKs, which are generated by the data processing between the terminal and the base transmission station.
Because the mobile station is designed to be mobile, in order to ensure mobility for wireless data calls, the mobile station often changes its transmission path when it moves. In order to determine the transmission path, the base transmission station ascertains the periodic change of the transmission path. However, determining the change of transmission path causes some trouble to the data transmission. Another disadvantage is that it cannot always change the transmission path at the right time.
The method of determining the transmission rate will be described as follows. If the mobile station uses a high speed data service, more call resources are used than with of the conventional IS95A and IS95B networks. The data flow between the mobile station and an upper PDSN is investigated. If a large amount of data is being transmitted, more call resources are assigned. If a lesser amount of data is being transmitted, the call resources are withdrawn so that the data transmission rate is controlled.
FIGS. 2a through 2d show handoff of a mobile station. FIG. 2a shows that a base transmission station 210 communicates with a mobile station 220 through FCH. FIG. 2b shows that SCH is additionally assigned while FCH is connected since the amount of data being transmitted has increased. FIG. 2c shows that a new PN with a target base transmission station 230 is added as the mobile station 220 moves from an area covered by the serving base transmission station 210 towards an area covered by the target base transmission station 230. FIG. 2d shows that the mobile station 220 moves to the target base transmission station 230, and the target base transmission station 230 reassigns the SCH between the mobile station 220 and the target base transmission station to another active PN. FCH can connect to a plurality of base transmission stations at the same time, while SCH can generally connect to only one base transmission station in general since wireless resources are wasted if SCH connects to a plurality of base transmission stations at the same time.
The process described above is called handoff or handover. The handoff of a high speed wireless data call includes the FCH handoff and SCH handoff. Each of them is performed as follows.
The FCH handoff takes place when the mobile station moves to a target base transmission station it is using data service from the serving base transmission station. If the pilot strength from the target base transmission station exceeds a predetermined threshold at the mobile station, the mobile station reports this to the serving base transmission station. The mobile station then connects both the serving base transmission station and the target base transmission station at the same time through the SCH. As the mobile station moves away from the serving base transmission station and gets closer to the target base transmission station, the pilot strength of the serving base transmission station decreases below a predetermined threshold and the mobile station disconnects itself from the serving base transmission station. The mobile station then connects to the target base station exclusively through FCH.
The SCH handoff takes place when the mobile station 220 moves to a target base transmission station 230 while the mobile station 220 is using data service provided by the serving base transmission station 210 as shown in FIGS. 2c and 2d. The mobile station 220 reports to a base transmission station 210 that the pilot strength of the serving base transmission station 210 has decreased and the pilot strength of the target base transmission station 230 has increased. The base transmission station compares the pilot strength of the serving base transmission station 210 and the pilot strength of the target base transmission station 230. If the pilot strength of the target base transmission station 230 is greater than the pilot strength of the serving base transmission station 210 by a predetermined amount, the base transmission station 210 reassigns SCH from the serving base transmission station 210 to the target base transmission station 230.
The ADD state occurs when a mobile station moves from a serving base transmission station to a target base transmission station and adds the target base transmission station. The DROP state occurs in handoff when a mobile station moves to a target base transmission station and drops the previous station. The SWAP state occurs when the ADD state and the DROP state occur simultaneously.
In high speed data service, the transmission rate is determined using the FCH and the SCH as the transmission path for the mobile station. If the FCH is assigned and the data flow increases, an additional wireless channel SCH is assigned. At the same time, as the mobile station that uses data service moves, a principal channel and an additional channel should be moved from the serving base transmission station (serving BTS) to the target base transmission station (target BTS). For this, the handoff should be performed to move the channel assigned to the mobile station from the serving base transmission station to the target base transmission station. The mobile station moves from the serving base transmission station to the target base transmission station while the mobile station is using a service of the serving base transmission station FIGS. 2c and 2d). The mobile station reports to a base transmission station that the pilot strength of the serving base transmission station has decreased and the pilot strength of the target base transmission station has increased at the mobile station. The base transmission station compares the pilot strength of the serving base transmission station and the pilot strength of the target base transmission station. In order to determine the appropriate timing for moving wireless resources from the serving base transmission station to the target base transmission station (FIG. 2d), the base transmission station reassigns a wireless channel from the serving base transmission station to the target base transmission station whose pilot strength is greater than that of the serving base transmission station.
When determining the transmission path, the pilot strength of the serving base transmission station is compared with the pilot strength of the target base transmission station, and a wireless channel is reassigned. However, comparison of pilot strengths alone does not ensure optimal data transmission. This is because the pilot strength provided to every mobile station through a pilot channel is affected by the wireless environment as well as other mobile stations which use data and voice service. Additionally, pilot strength is not exactly proportional to the Frame Error Rate (FER) of a traffic channel through which the mobile station directly uses data service. Using comparison of pilot strengths to reassign wireless channels deteriorates data transmission efficiency due to frequent reassigning since the pilot strength can vary frequently.
FIG. 3 is a flow chart illustrating a method for assigning and withdrawing call resources to determine data transmission rate when transmitting and receiving data in the system of FIG. 1. First, the mobile station requests the base transmission station to provide a high speed data service using PPP (S121). The base transmission station then determines whether the request to provide service is proper (S122).
If the service request is proper, the base transmission station performs a wireless call setup procedure (S123), and then performs PPP communication with an upper PDSN. As a result, the mobile station is given an IP to connect to data networks (S124). The mobile station connects to an external data network, requests data and receives the requested data, (S125).
In this case, if the amount of the data requested by the mobile station exceeds a predetermined amount, the base transmission station assigns as additional wireless channel to increase the transmission rate so that data transmission is more effective (WAIT UP) (126). If all of the data requested by the mobile station is received, or the data request is stopped, the additional wireless channels are all withdrawn (WAIT DOWN) (S127). However, when the data flow between the mobile station and the upper PDSN is checked and additional wireless channels are assigned or withdrawn, the channels are assigned or withdrawn without taking into account the effect of the wireless environment.
In other words, the mobile station uses the wireless link protocol (RLP: Radio Link Protocol). This protocol is based on sequential data transmission and is a NAK based protocol which requests retransmission if the data is not received normally, due to either the wireless environment or other reasons.
If the mobile station uses data service and retransmissions occur frequently, data transmission may not be performed normally. The data transmission is decreased by controlling the data flow between an external data network server and the mobile station if data transmission is not performed normally. Additionally, the base transmission station withdraws the corresponding wireless channels.
A disadvantage of the related art is that is takes too much time to withdraw channels. If retransmission does not continue to occur, or does not occur in a short amount of time, the data transmission rate is decreased through flow control even though the wireless environment improves. The method of assigning and withdrawing wireless channels by checking the data flow is not in keeping with the nature of the wireless environment, which varies frequently. Consequently, the optimal data transmission rate suitable to the wireless environment cannot be found. The related art has a problem in that the optimal transmission rate and transmission path are not provided, since the wireless environment is not considered when determining the transmission rate or the transmission path for providing a high speed data service to a mobile station.
The method of determining the transmission rate and the transmission path in a mobile communication system for packet data service in the related art will now be described.
First, the method of determining the transmission rate in the related is described. A mobile station that uses IS95C high speed data service requires more call resources than a mobile station using IS95A and IS95B data service. Accordingly, the data flow between the mobile station and an upper PDSN is checked. More call resources are assigned when there is a large amount of data. The call resources are withdrawn when there is a small amount of data. The data transmission rate is controlled manually.
Second, the method of determining the transmission path in the related art is described. In IS95C data service, data transmission rate is determined using a fundamental channel (FCH) and a supplemental channel (SCH). As shown in FIGS. 2a and 2b, if data flow is increased after assigning FCH, SCH is assigned in addition to FCH. As shown in FIGS. 2c and 2d, as the mobile station that uses data service moves, FCH and SCH are moved from the serving base transmission station assigned beforehand to the target base transmission station that the mobile station moves to.
In order to control the data transmission rate both up and down, the following is defined. The supplemental channel SCH is assigned to use CDMA 2000 1 X packet data service. According to the speed of SCH, the SCH occupies 0, 1, 2, 4, 8 and 16 channel resources. Each of these speeds is expressed as follows: SCHR—0X, SCHR—1X, SCHR—2X, SCHR—4X, SCHR—8X, and SCHR—16. Accordingly, the current SCH rate (SCHR) is determined to be any one of six. The reference to increase the transmission rate from current SCH rate to an upper speed is up-threshold. The reference to decrease the transmission rate from current SCH rate to a lower speed is down-threshold. In other words, if the amount of the data exceeds the up-threshold, the transmission rate is increased to an upper speed. If the amount of the data falls below the down-threshold, the transmission rate is decreased to the lower speed.
However, the above-mentioned method of setting the data transmission rate and the transmission path in the related art has various problems.
For example, in determining transmission rate, the base transmission station checks the data flow between a mobile station and a PDSN and makes a determination to either assign additional wireless channels or withdraw wireless channels. However there is a problem with the method described above in that the reference to assign additional wireless channels or withdraw wireless channels is based simply on data flow, without considering the influence of the wireless environment of the present situation. The mobile station uses RLP (Radio Link Protocol) for high speed data service. This is a retransmission based protocol (NAK Based Protocol) which makes a request for retransmission if the data not normally transmitted due to either the wireless environment or other causes. If a large number of NAK messages are generated while the base transmission station provides some data service to the mobile station, data transmission is not performed normally. If the data transmission is not performed normally, as described above, the data transmission is decreased by flow control imposed between an external network server and the base transmission station. The base transmission station then withdraws wireless channels. This makes the channel withdrawal process take too much time.
Additionally, if retransmission does not continue to occur, or does not occur in a short amount of time, the data transmission rate is decreased even though the wireless environment improves at the time to withdraw the wireless channel through flow control. In other words, the method of assigning and withdrawing wireless channels by checking the data flow is not in keeping with the nature of the wireless environment, which varies frequently. Consequently, the optimal data transmission rate suitable to the wireless environment cannot be found.
Moreover, when determining the transmission path, the pilot strength of the serving base transmission station is compared with the pilot strength of the target base transmission station and a wireless channel is reassigned. However, comparison of pilot strengths alone does not ensure optimal data transmission. This is because the pilot strength provided to every mobile station through a pilot channel is affected by the wireless environment, as well as other mobile stations which use data and voice service.
Additionally, pilot strength is not exactly proportional to the FER (Frame Error Rate) of a traffic channel through which the mobile station directly uses data service. The method of reassigning a wireless channel by using only the pilot strengths deteriorates data transmission efficiency due to frequent reassigning, as pilot strength varies frequently.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.